Lecture Notes: Cardiac Action Potential

Introduction to Cardiac Muscle

• Cardiac Muscle: Located in the heart, responsible for pumping blood.
  • Average heart rate: 70 beats per minute.
  • Over 100,000 contractions per day.
  • Must adapt to physiological and pathological changes.

Cardiac Action Potentials

• Myocytes: Cells of the heart.
• Types of Action Potentials:
  • Non-Pacemaker Action Potential (Fast Response)
    • Found throughout the heart except pacemaker cells.
    • Responsible for rapid depolarization.
  • Pacemaker Action Potential (Slow Response)
    • Found in sinoatrial and atrioventricular nodes.
    • Slower rate of depolarization.
• Differences from Neural and Skeletal Muscle Action Potentials:
  • Duration: Nerve (1 ms), Skeletal Muscle (2-5 ms), Cardiac (200-400 ms).

Non-Pacemaker Action Potential

• Resting Membrane Potential: -90 mV.
• Ion Movement: Potassium leaks out, sodium leaks in.
• Action Potential Phases:
  • Phase 4: Resting potential.
  • Phase 0: Depolarization; sodium enters, membrane becomes positive.
  • Phase 1: Early repolarization; potassium exits.
  • Phase 2: Plateau stage; calcium enters, potassium continues to exit.
  • Phase 3: Repolarization; potassium exits, returns to resting potential.
• Refractory Period: Prevents tetany, ensures muscle relaxation.

Pacemaker Action Potential

• Automaticity: Ability of heart to beat in unison.
• Pacemaker Cells:
  • Sinoatrial Node
  • Atrioventricular Node
  • Bundle of His/Purkinje Fibers
• Phases:
  • Phase 4: Resting potential.
  • Phase 0: Depolarization; sodium and calcium enter.
  • Phase 3: Repolarization; potassium exits.
• Differences: No stable resting potential, spontaneous depolarization.

Mechanisms of Ion Exchange

• Sodium: Enters through sodium-gated channels, increases membrane potential.
• Calcium: Enters during depolarization, essential for contraction.
• Potassium: Exits to repolarize the cell, returns to resting potential.

Role of Catecholamines

• Reuptake of Calcium: Assisted by catecholamines.
• Phosphorylation of Myosin: Increases cross-bridging rate and calcium reuptake.
• Relaxation: Facilitated by catecholamines, aids in recovery of cardiac muscle.

Conclusion

• Understanding cardiac action potentials is crucial for comprehending heart function and its regulation.
• Key role of ion exchange and electrical activity in heart contractions.

Thank you for listening.